The use of mobile communications networks has increased over the last decade. Operators of the mobile communications networks have increased the number of base stations in order to meet an increased demand for service by users of the mobile communications networks. The operators of the mobile communications network wish to reduce the running costs of the base station. One option to do this is to implement a radio system as an antenna-embedded radio forming an active antenna array. Many of the components of the antenna-embedded radio may be implemented on one or more chips.
Nowadays active antenna arrays are used in the field of mobile communications systems in order to reduce power transmitted to a handset of a customer and thereby increase the efficiency of the base station, i.e. the radio station. The radio station typically comprises a plurality of antenna elements, i.e. an antenna array adapted for transceiving a payload signal. Typically the radio station comprises a plurality of transmit paths and receive paths. Each of the transmit paths and receive paths are terminated by one of the antenna elements. The plurality of the antenna elements used in the radio station typically allows the steering of a beam transmitted by the antenna array. The steering of the beam includes but is not limited to at least one of: detection of direction of arrival (DOA), beam forming, down tilting and beam diversity. These techniques of beam steering are well-known in the art.
The code sharing and time division strategies as well as the beam steering rely on the radio station and the antenna array to transmit and receive within well defined limits set by communication standards. The communications standards typically provide a plurality of channels or frequency bands useable for an uplink communication from the handset to the radio station as well as for a downlink communication from the radio station to the handset. In order to comply with the communication standards it is of interest to reduce so called out of band emissions, i.e. transmission out of a communication frequency band or channel as defined by the communication standards.
For example, the communication standard “Global System for Mobile Communications (GSM)” for mobile communications uses different frequencies in different regions. In North America, GSM operates on the primary mobile communication bands 850 MHz and 1900 MHz. In Europe, Middle East and Asia most of the providers use 900 MHz and 1800 MHz bands.
Typically, a remote radio head or active antenna array is designed to transmit a single band. Digital dividend spectrum auctions and other releases of frequency spectrum have led to the desire from operators for multi-band products. Multi-band products save space on masts and hence save site rental and installation costs. The multi-band products may also enable two bands to be accommodated on heavily used masts where no space exists for additional antennas.
Multi-band transmitters for remote radio heads or active antenna arrays have been developed. FIG. 1 shows an example of such a conventional multi-band transmitter 1′, in which a hybrid combiner is used for combining the outputs of two separate single-band transmitters.
The transmitter 1′ as shown in FIG. 1 comprises two single-band transmitters 1A, 1B. The single-band transmitter 1A, 1B comprises a transmit path 1000-A, 1000-B between a digital signal processor 15 and a combiner 7. A payload signal S is processed by the digital signal processor 15, for example undergoing filtering, upconversion, crest factor reduction and beamforming processing, prior to forwarding to a digital-to-analogue conversion block 2-1, 2-2 adapted to convert the payload signal 2000 into an analogue payload signal SA, SB as a transmit signal.
The digital-to-analogue conversion block 2-1, 2-2 of FIG. 1 comprises delta-sigma digital-to-analogue converters. The analogue signal SA, SB is passed to a first filter 3-1, 3-2. Typically, the first filter 3-1, 3-2 comprises a band pass filter. The first filter 3-1, 3-2 allows the analogue payload signal SA, SB to pass the first filter 3-1, 3-2 in a group of frequency bands or channels as defined by the communication standard, such as 3GPP. The purpose of the first filter 3-1, 3-2 is to remove unwanted products from the digital to analogue conversion process, such as noise or spurious signals.
The first filter 3-1 in the transmit path 1000-A may pass the frequencies corresponding to a first transmit frequency band TB1. The first filter 3-2 in the transmit path 1000-B may pass the frequencies corresponding to a second transmit frequency band TB2. The first band TB1 may correspond to the frequency range according to an 800 MHz band Long Term Evolution LTE downlink (base-station transmit) communications frequency plan and the second band TB2 may correspond to the frequency range according to a GSM downlink—base-station transmit—communication frequency plan, e.g. 925-960 MHz.
The output of the first filter 3-1, 3-2 is passed to a radio frequency amplifier 4-1, 4-2 followed by a second filter 5-1, 5-2. The purpose of the second filter 5-1, 5-2 is to remove unwanted products from the amplification.
The outputs of the second filter 5-1, 5-2 on the transmit paths 1000-A, 1000-B are passed to a coupler 6-1, 6-2. The coupler 6-1, 6-2 is adapted to extract a portion of the transmit signal SA, SB as feedback signal SAF, SBF for correcting nonlinearities introduced by the amplifier 4-1, 4-2, as will be explained later.
The outputs of the coupler 6-1, 6-2 are the outputs of the single band transmitters 1A, 1B, which are combined in a broadband combiner 7. The broadband combiner 7 is adapted to combine the signal SA, SB on the two transmit paths 1000-1, 1000-2 into a combined signal SC. The broadband combiner 7 may be a conventional hybrid combiner or a Wilkinson combiner, as known in the art.
The amplifier 4-1, 4-2 typically introduces nonlinearities into the transmit path 1000-A, 1000-B. The nonlinearities introduced by the amplifier 4-1, 4-2 affect transfer characteristics of the transmit path. The nonlinearities introduced by the amplifier 4-1, 4-2 distort the payload signal relayed by the radio station as a transmit signal along the transmit paths.
The transfer characteristics of a device describe how the input signal(s) generate the output signal. It is known in the art that the transfer characteristics of a nonlinear device, for example a diode or an amplifier, are generally nonlinear.
The concept of predistortion uses the output signal of a device, for example from the amplifier, for correcting the nonlinear transfer characteristics of the device. The output signal of the device is compared to the input signal of the device by means of feedback and from this comparison correction coefficients are generated which are used to form or update an “inverse distortion” or predistortion signal. The predistortion signal is added and/or multiplied to the input signal in order to linearise the transfer characteristics of the device. The nonlinear transfer characteristics of the device can be corrected by carefully adjusting the predistortion.
To apply a correct amount of the predistortion signal to the amplifier it is of interest to know the distortions or nonlinearities introduced by the amplifier. This is commonly achieved by the feedback of the transmit signal to a predistorter. The predistorter is adapted to compare the transmitted signal (output signal) with a signal prior to amplification (input signal) in order to determine the distortions introduced by the amplifier. The input signal prior to amplification is, for example, the payload signal.
The concept of the predistortion has been explained in the above description in terms of correcting the transfer characteristics with respect to the amplitude of the transmit signal. It will be understood that predistortion may alternatively and/or additionally correct for nonlinearities with respect to a phase of the input signal and the output signal.
The nonlinearities of the transfer characteristics of the complete transmit path from a digital signal processor to the antenna element are typically dominated by the nonlinearities in the transfer characteristics of the amplifier. It is therefore often sufficient to correct for the nonlinearities of the amplifier.
Referring to FIG. 1, the feedback signal SAF, SBF is extracted in order to correct the nonlinearities of the amplifier 4-1, 4-2. The feedback signal 2AF, 2BF is passed on a feedback path 1000-AF, 1000-BF leading to a digital predistortion unit 11, as is known in the art.
The feedback path 1000-AF, 1000-BF comprises an attenuator 8-1, 8-2, a filtering and down conversion block 9-1, 9-2, followed by an analogue-to-digital converter 10-1, 10-2. The digitized feedback signals 2AF, 2BF are used by the digital predistortion unit 11 to extract the nonlinearities introduced by the amplifier 4-1, 4-2 and correct the transmit signal.
As will be readily apparent to the man skilled in the art, the conventional dual band transmitter 1′ comprises combining the outputs of two separate single-band transmitters 1A, 1B, using a broadband coupler such as a Wilkinson or 3 dB hybrid coupler 7. The combiner 7 however introduces a minimum of 3 dB of loss into each of the RF output paths and therefore wastes valuable RF output power. The conventional dual band transmitter 1′ is however flexible and can be used to combine almost any radio frequency bands to feed a single antenna or antenna element.
FIG. 2 shows another conventional multi-band transmitter 1″, which utilises a filter-based combiner 7A, 7B. The multi band transmitter of FIG. 2 differs from FIG. 1 in that the broadband combiner 7 is replaced by two duplexers 7A, 7B for combining the outputs of the two separate single-band transmitters 1A′, 1B′. Those elements of FIG. 2 which are identical to the elements of FIG. 1 have identical reference numerals.
The dual band transmitter 1″ of FIG. 2 advantageously reduces the loss compared to the dual-band transmitter 1′ of FIG. 1, because the duplexers 7A, 7B induce less loss than the broadband combiner 7. However, the multi-band transmitter 1″ of FIG. 2 is also less flexible than the multi-band transmitter 1′ of FIG. 1 based on the hybrid combiner approach. The two duplexers must indeed be designed specifically for each pair or multiplicity of bands to be combined.